Direct and indirect effects of simulated calcareous dredge material on eggs and larvae of pink snapper Pagrus auratus

The direct and indirect effects of a simulated, calcarenite‐based dredge material on eggs and larvae of pink snapper Pagrus auratus were assessed. Direct effects were assessed by measuring hatch rate or survival of eggs and pre‐feeding larvae, respectively, over a range of concentrations and exposure durations. Exposure of eggs to suspended solid concentrations up to 10 000 mg l^?1 for 24 h did not affect egg buoyancy or hatch rate, despite sediment adherence occurring at the two highest concentrations tested. Newly hatched larvae, whose mouths were still closed, were relatively tolerant of suspended solids, with a 12 h lethal concentration resulting in 50% mortality, LC₅₀, of 2020 mg l^?1 and a first observable effect concentration of 150 mg l^?1. Once the larvae's mouths opened, tolerance was significantly reduced, with a 12 h LC₅₀ of 157 mg l^?1 and a first observable effect concentration of 4 mg l^?1. Tolerance of larvae to suspended solids was negatively correlated with suspended solids concentration and exposure time, with exposure durations of ≤6 h being significantly less detrimental than those of 9 h or more. Indirect effects to larvae were assessed by measuring ingestion of copepod nauplii by 10 and 15 days post‐hatch (dph) larvae at sediment concentrations from 0 to 200 mg l^?1 in 50 mg l^?1 increments over 4 h. Ingestion was not significantly affected by sediment for 10 dph larvae, but by 15 dph, sediment had a far greater impact on ingestion, with larvae in all sediment treatments eating significantly fewer copepods than those in the control.     

Metabolic rate in diapause and nondiapause brown locust eggs correlated with embryonic development

Insects use dormancy to survive adverse conditions. Brown locust Locustana pardalina (Walk.) eggs offer a convenient model to study dormancy (diapause and quiescence), which contributes to their survival under arid conditions. The metabolic rates of developing nondiapause, diapause and quiescent eggs are compared in the present study using closed‐system respirometry. The embryo becomes committed to continue development and hatch or to enter diapause 6 days after the eggs are placed on moist soil. The metabolic rate of nondiapause eggs increases exponentially until hatching, whereas that of diapause eggs is low and stable. The metabolic rate of diapause laboratory eggs (1.9 ± 0.6 µL CO₂ mg^?1 h^?1) is significantly higher than that of field eggs (0.5 ± 0.3 µL CO₂ mg^?1 h^?1), although the ranges of metabolic rate overlap and the embryos are all in late anatrepsis. The metabolic rate of quiescent eggs is similar to that of diapause eggs but decreases with time. Low metabolic rates during arrested development allow eggs to persist over long periods before hatching.     

Toxicity of common biocides used in aquaculture to embryos and larvae of Brachymystax tsinlingensis Li

Brachymystax tsinlingensis Li is a threatened fish species endemic to China. With the problems of environmental factors and seeding breeding diseases, it is important to further improve the efficiency of seeding breeding and the basis of resource protection. This study investigated the acute toxicity of copper, zinc and methylene blue (MB) on hatching, survival, morphology, heart rate (HR) and stress behaviour of B. tsinlingensis. Eggs (diameter: 3.86 ± 0.07 mm, weight: 0.032 ± 0.004 g) of B. tsinlingensis were selected randomly from artificial propagation and developed from eye-pigmentation-stage embryos to yolk-sac stage larvae (length: 12.40 ± 0.02 mm, weight: 0.03 ± 0.001 g) and exposed to different concentrations of Cu, Zn and MB for 144 h in a series of semi-static toxicity tests. The acute toxicity tests indicated that the 96-h median lethal concentration (LC₅₀) values of the embryos and larvae were 1.71 and 0.22 mg l⁻¹ for copper and 2.57 and 2.72 mg l⁻¹ for zinc, respectively, whereas the MB LC₅₀ after 144-h exposure for embryos and larvae were 67.88 and 17.81 mg l⁻¹, respectively. The safe concentrations of copper, zinc and MB were 0.17, 0.77 and 6.79 mg l⁻¹ for embryos and 0.03, 0.03 and 1.78 mg l⁻¹ for larvae, respectively. Copper, zinc and MB treatments with concentrations greater than 1.60, 2.00 and 60.00 mg l⁻¹, respectively, led to a significantly low hatching rate and significantly high embryo mortality (P < 0.05), and copper and MB treatments with concentrations greater than 0.2 and 20 mg l⁻¹ led to significantly high larvae mortality (P < 0.05). Exposure to copper, zinc and MB resulted in developmental defects, including spinal curvature, tail deformity, vascular system anomalies and discolouration. Moreover, copper exposure significantly reduced the HR of larvae (P < 0.05). The embryos exhibited an obvious change in behaviour, converting from the normal behaviour of emerging from the membrane head first to emerging tail first, with probabilities of 34.82%, 14.81% and 49.07% under copper, zinc and MB treatments, respectively. The results demonstrated that the sensitivity of yolk-sac larvae to copper and MB was significantly higher than that of embryos (P < 0.05) and that B. tsinlingensis embryos or larvae might be more resistant to copper, zinc and MB than other members of the Salmonidae family, which benefits their resource protection and restoration.     

Sublethal Effects of Nitrite on Eastern Tiger Salamander (<Emphasis Type="Italic">Ambystoma tigrinum tigrinum</Emphasis>) and Wood Frog (<Emphasis Type="Italic">Rana sylvatica</Emphasis>) Embryos and Larvae: Implications for Field Populations

Ephemeral pools, which can have high animal biomass and low dissolved oxygen, may be prone to nitrite accumulation. As such, it is important to understand how exposure to nitrite might affect development and growth of amphibians that breed in these ephemeral pools. Wood frog (Rana sylvatica) and eastern tiger salamander (Ambystoma tigrinum tigrinum) embryos and tadpoles and young larvae were exposed to elevated concentrations of nitrite derived from sodium nitrite: 0, 0.3, 0.6, 1.2, 2.1, 4.6, and 6.1 mg l⁻¹ NO₂–N. Increasing nitrite exposure slowed embryonic and larval development in both the eastern tiger salamander and the wood frog, reduced growth in tiger salamander embryos and larvae, and delayed metamorphosis in the wood frog. At concentrations less than 2 mg l⁻¹ NO₂–N nitrite delayed hatching, and at concentrations above 2 mg l⁻¹ time to hatching decreased causing more individuals to hatch at less developed stages. Nitrite also increased asynchrony in tiger salamander hatching. The sublethal effects of nitrite on amphibian development, growth and hatching could have serious repercussions on amphibian fitness in ephemeral environments. Potential increases in mortality on field populations caused by sublethal effects of nitrite are discussed.     

Acute toxic effects of 8‐epidiosbulbin E,a 19‐norclerodane diterpene from yam Dioscorea antaly,on medaka Oryzias latipes embryos

The fractionation of an aqueous extract of yam Dioscorea antaly from Madagascar led to the isolation of terpenoids and flavonoids. Compounds were identified on the basis of modern mass spectrometry and two‐dimensional nuclear magnetic resonance (2D‐NMR). Toxicological effects of the most abundant isolated compound, 8‐epidiosbulbin E were studied on medaka Oryzias latipes embryo‐larval development. The lethal concentration (killing 50%; LC₅₀) to embryos treated 24 h before hatching and for 3 days after hatching was estimated to be 0·56 mg ml^?1 (P< 0·05). No mortality was observed with O. latipes larvae exposed after hatching until day 4. Anatomo‐pathological studies of embryos exposed to 0·56 mg ml^?1 showed development anomalies of the central nervous system, liver, muscle and intestine. The present data thus extend the model of O. latipes embryos as a useful animal model to analyse the effects of food toxins.     

Sublethal effects of carbaryl on embryonic and gonadal developments of zebrafish Danio rerio

Carbaryl is a broad-spectrum insecticide used to control insect pests. In aquatic environments, it can disrupt the endocrine system and adversely affect the reproductive function of aquatic animals. This study investigated sublethal impacts of carbaryl on embryos and gonads of zebrafish Danio rerio in order to assess the pesticide's impact on its reproduction. Fertilised embryos were exposed to 1.7 mg l^–1 carbaryl until hatching, while larvae aged 10 days post hatching were exposed to 0.1, 0.2, 0.4, 0.8 and 1.7 mg l^–1 carbaryl concentrations until 50 days post hatching (dph). Treatments were applied in a static renewal system and all experiments involved water only and a solvent control. At the end of 50 dph, all surviving fish were sacrificed and processed for light microscopy. Results indicated a mean hatching success rate of 92.5% for control groups, while embryos exposed to carbaryl recorded an 81.0% success rate. Sex reversal was delayed in the experimental groups, with a sex ratio of 13 females to 0 males, but the control group recorded 6 females to 8 males. These results suggest that sublethal doses of carbaryl in the environment, similar to those used in the current study, may have an adverse effect on the reproductive success of zebrafish.     

Disturbances due to increased salinity and the resilience of zooplankton communities: the potential role of the resting egg bank

When faced with disturbances such as increased salinity, aquatic communities inhabiting inland coastal systems change and may or may not be resilient after salinity decreases. The purpose of this study was to assess the potential role of the resting egg bank for zooplankton community resilience. We predicted that (1) hatching of resting eggs is inhibited by increased salinities and (2) resting eggs remain viable when exposed to salinity and hatch when returned to freshwater. At the community level, we evaluated the hatching responses and the short-term viability of resting eggs exposed to a salinity gradient. The hatching of resting eggs was inhibited at higher salinities (16.0 and 32.0 g l^?1). However, some resting eggs remained viable and hatched when returned to freshwater. Additionally, combining our experimental results to previously published field data, we observed that the pattern of hatching during exposure to salinity matches the temporal succession observed in the zooplankton community at our model system, after increased salinity. The recovery of zooplankton communities after disturbances involving increased salinity is likely facilitated by the presence of an egg bank. This finding has important implications for the recovery of zooplankton communities and the management of aquatic systems vulnerable to salinization worldwide.     

Rearing of Procypris rabaudi during early life-history stages

The early life‐history of Chinese rock carp Procypris rabaudi was investigated during a 56‐day rearing period: 318 artificially propagated P. rabaudi larvae were reared throughout metamorphosis in a small‐scale recirculation system (345 L water volume, 10 × 18 L rearing tanks, 150 L storage and filter compartment with bioballs, 20–30 larvae L^?1) at the Institute of Hydrobiology, Wuhan, Hubei Province, China. The newly hatched larvae had an initial total length of 8.93 ± 0.35 mm SD (n = 10) at 3 days post‐hatch and reached an average total length of 33.29 mm (±1.88 mm SD, n = 10) 56 days after hatching. Length increment averaged 0.45 mm day^?1, resulting in a mean growth of 24.4 mm within the 56‐day period. High mortality rates of up to 92% derived from an introduced fungus infection and subsequent treatment stress with malachite green. Our results indicate that Chinese rock carp can be raised successfully from artificially fertilized eggs. We therefore assume this species to be a candidate for commercial aquaculture.     

Plasticity of hatching in amphibians: evolution, trade-offs, cues and mechanisms

Many species of frogs and salamanders, in at least 12 families, alter their timing of hatching in response to conditions affecting mortality of eggs or larvae. Some terrestrially laid or stranded embryos wait to hatch until they are submerged in water. Some embryos laid above water accelerate hatching if the eggs are dehydrating; others hatch early if flooded. Embryos can hatch early in response to predators and pathogens of eggs or delay hatching in response to predators of larvae; some species do both. The phylogenetic pattern of environmentally cued hatching suggests that similar responses have evolved convergently in multiple amphibian lineages. The use of similar cues, including hypoxia and physical disturbance, in multiple contexts suggests potential shared mechanisms underlying the capacity of embryos to respond to environmental conditions. Shifts in the timing of hatching often have clear benefits, but we know less about the trade-offs that favor plasticity, the mechanisms that enable it, and its evolutionary history. Some potentially important types of cued hatching, such as those involving embryo-parent interactions, are relatively unexplored. I discuss promising directions for research and the opportunities that the hatching of amphibians offers for integrative studies of the mechanisms, ecology and evolution of a critical transition between life-history stages.     

Developmental response of the beneficial predator Podisus maculiventris to change in dietary ascorbic acid concentration

We report here the effects of a range of ascorbic acid concentrations (0.07, 0.3, 3.0, and 30.0 g l^?1) in artificial diets on growth rates, adult weights, fecundity, and survival over two generations of the predatory stink bug, Podisus maculiventris (Say) (Heteroptera: Pentatomidae). Overall, a dietary level of 3.0 g l^?1 gave the shortest developmental times over two generations. The likelihood of egg hatch at one ascorbic acid concentration compared to another concentration suggested that egg hatch increased as the concentration of ascorbic acid increased from 0.07 to 3.0 g l^?1 and then declined from 3.0 to 30.0 g l^?1. The combination of the maximum egg oviposition at 0.3 and 3.0 g l^?1, egg hatch at 3.0 g l^?1, and survival at 0.07 and 0.3 g l^?1 suggests an overall superior performance at a concentration between 0.3 and 3.0 g l^?1. Depletion of ascorbic acid below 3.0 g l^?1 or addition of ascorbic acid above 3.0 g l^?1 lowered the likelihood of egg hatch, which became more pronounced in the second generation. This is consistent with previously published information for phytophagous insects.     

The influence of ionic concentration on population parameters,development time,activity, and respiration rate of Arctodiaptomus spinosus (Daday) (Calanoida,Copepoda)

Summary For A. spinosus, significant correlations were found between population parameters and the ecological parameters of alkalinity and temperature. For the birth rate, 43.33% of variance is explained by alkalinity and 56.33% by alkalinity and temperature. Alkalinity, and alkalinity and temperature combined account for 48.22% and 70.23% respectively of the observed variances in nauplii hatching. It was demonstrated that, provided sufficient time for adaptation is allowed, the activity of individuals is not affected by differences in the alkalinity of the medium within the normal range of concentration. Embryonic development does not appear to be influenced by the ionic concentration, but significant increases in time of development of the nauplii (at alkalinity 100 meq/l: 8.8 days/20°C) and of the copepodites (at alkalinity 100 meq/l: 12.6 days/20°C) were recorded at both low and high ionic concentrations. The lowest respiration rates (3.7 nl O₂·h^-1·g dw^-1/20°C) occurred at an alkalinity of 100 meq/l. Rates increased at lower (10 meq/l: 5.7 nl) and at higher (400 meq/l: 11.9 nl) alkalinities.     

Effects of dietary astaxanthin supplementation on reproductive characteristics of rainbow trout (Oncorhynchus mykiss)

The effect of dietary astaxanthin supplementation on reproductive characteristics was investigated in five groups of female rainbow trout broodstock fed diets containing either 0.07, 12.46, 33.33, 65.06 or 92.91 mg astaxanthin kg^?1, respectively, and two groups of male rainbow trout broodstock fed diets supplemented with 0.07 and 33.33 mg astaxanthin kg^?1, respectively, for 6 months in an artificial photoperiod system until sexual maturation. The eggs from each group of female broodstock were divided into two equal batches. One batch was fertilized with homogenized sperm of four males fed diets with 0.07 mg astaxanthin kg^?1 and the other portion with sperm of four males fed diets with 33.3 mg astaxanthin kg^?1. The females produced eggs with astaxanthin concentrations ranging from 2.03 to 29.79 mg kg^?1. Dietary astaxanthin supplementation had positive effects on investigated reproductive traits. Significant differences in rate of fertilization, percentage of eyed and hatched eggs, and mortality of eyed eggs were found between treatments (P < 0.05), but no significant difference was found on percentage of mortality before hatching (P > 0.05). A significant difference (P < 0.05) in fertilization rate was found for male groups fed 0.07 and 33.3 mg astaxanthin kg^?1. The astaxanthin content in the eggs and fertilization rate, eyed‐egg percentage and percentage hatch were significantly correlated (P < 0.05). It is concluded that dietary supplements of astaxanthin are required for optimum reproduction in rainbow trout.     

Early ontogeny of Ancherythroculter nigrocauda and effects of delayed first feeding on larvae

Development of embryos and larvae in Ancherythroculter nigrocauda Yih et Woo (1964) and effects of delayed first feeding on larvae were observed after artificial fertilization. The fertilized eggs were incubated at an average temperature of 26.5°C (range: 25.7–27) and the larvae reared at temperatures ranging from 21.8 to 28°C. First cleavage was at 50 min, epiboly began at 7 h 5 min, heartbeat reached 72 per min at 24 h 40 min and hatching occurred at 43 h 15 min after insemination. Mean total length of newly hatched larvae was 4.04 ± 0.03 mm (n = 15). A one‐chambered gas bladder was observed at 70 h 50 min, two chambers occurred at 15 days, and scales appeared approximately 30 days after hatching. Larvae began to feed exogenously at day 4 post‐hatch at an average temperature of 24°C. Food deprivation resulted in a progressive atrophy of skeletal muscle fibres, deterioration of the larval digestive system and cessation of organ differentiation. Larval growth under food deprivation was significantly affected by the time of first exogenous feeding. Starved larvae began to shrink, with negative growth from day 6 post‐hatch. The point of no return (PNR) was reached at day 11 after hatching. Mortality of starved larvae increased sharply from day 12 after hatching.     

Effects of sodium chloride, formalin and iodine on the hatching success of common carp, Cyprinus carpio, eggs

Saprolegniales are ubiquitous in natural water supplies of fish hatcheries, and often cause serious disease problems. Sodium chloride, formalin and iodine, administered twice a day as a flush at different concentrations, were tested on infected eggs of common carp, Cyprinus carpio, to evaluate their antifungal activity and effect on hatching rates. Sodium chloride at 35 000 mg L^?1 and formalin at 400 mg L^?1 were found to be most effective in controlling Saprolegnia sp., with 85.4 and 91.8% hatching rates, respectively. Iodine increased the hatching rate by 27% at 200 mg L^?1 (P < 0.05). There were infections on eggs exposed to all levels of iodine, but not on eggs treated with sodium chloride and formalin. Sodium chloride and formalin were more effective than iodine in controlling Saprolegnia. Sodium chloride is a safe, efficacious and economical treatment of Saprolegniosis and is therefore recommended for treating common carp eggs.     

The effect of different kinds of electrolyte and non‐electrolyte solutions on the survival rate and morphology of zebrafish Danio rerio embryos

The effect of electrolyte and non‐electrolyte solutions on the survival and on the morphology of zebrafish Danio rerio embryos was investigated. Embryos in different ontogenetic stages were incubated in electrolyte (NaCl, KCl, MgCl₂ and CaCl₂) and non‐electrolyte solutions [sucrose and polyvinylalcohol (PVA)] of different concentrations for 5 – 15 min. The embryos were hatched to the long‐pec stage and the effective concentrations which caused a 50% decrease in embryo development (EC₅₀) were determined. The morphometric changes, which were caused by the test solutions, were measured. Ion channel blockers were used to see if active ion transport played a role for embryo survival. Finally, dechorionated embryos were exposed to the test solutions to get indications about the importance of chorion and perivitelline space. For 12 hours post fertilization (hpf) embryos and a 15 min exposure period, EC₅₀ was highest for MgCl₂ (1·60 mol l^?1), followed by sucrose (0·73 mol l^?1), NaCl (0·49 mol l^?1), KCl (0·44 mol l^?1), CaCl₂ (0·43 mol l^?1) and PVA [0·0005 mol l^?1 (2·2%)]. EC₅₀ were lower for early embryonic stages than for advanced stages for all solutions with exception of MgCl₂ and sucrose. At the EC₅₀, MgCl₂ and CaCl₂ solutions did not induce morphometric changes. NaCl and sucrose solutions induced reversible morphometric changes, which were compensated within 10 min. Only the EC₅₀ of KCl and PVA solutions induced permanent morphometric changes, which could not be compensated. Incubation of embryos in electrolyte and non‐electrolyte solutions together with ouabain (blocker of Na⁺– K⁺ ATPase), HgCl₃ (dose‐dependent inhibition of aquaporine channels), verapamil (inhibition of calcium and magnesium uptake) and amiloride (inhibition of sodium uptake) significantly decreased the per cent of embryos developing to the long‐pec stage in comparison to the same solutions without blockers. Ouabain and HgCl₃ also induced morphometric changes. For dechorionated embryos the survival rates in water and in the different test solutions were similar to untreated embryos.     

To hatch or not to hatch? Egg hatch response to larval density and to larval contact in a treehole mosquito

Abstract.

• 1 We investigated the effect on egg hatch of exposure to: (1) varying larval density, and (2) larval contact in Aedes triseriatus Say (Diptera: Culicidae). For 2 days in the laboratory we submerged eggs into a treehole water medium containing 0 (control), 4, 12 or 24 larvae that could either contact the eggs directly or were separated from them by a screen. Following treatment, abundance of microorganisms on the egg surfaces, a food source for newly hatched larvae and a proposed hatching stimulus, was assessed by counts made from serial dilutions of samples.
• 2 We discovered a complex hatching response to larval contact and to larval density, and an interaction between these two factors in their effect on microbial growth. Hatching was inhibited in the 0-larva control, even though microorganisms grew abundantly on the eggs. Hatch rate, as well as microbial counts, were high for eggs in direct contact with 4 larvae. As density increased in the larval contact treatment, microorganisms disappeared from the egg surfaces and hatch rate declined.
• 3 When protected from larval grazing, eggs supported numerous microbial colonies irrespective of larval density. In contrast to the contact treatment group, egg hatch increased with increasing larval density. These observations suggest that the combination of microbial growth and a larval factor stimulates hatch. This hatching response may have evolved because both abundant microorganisms and numerous larvae reflect a habitat of good quality.

     

The hatching time of Locusta migratoria under outdoor conditions: role of temperature and adaptive significance

The present study investigates the time of hatching of the migratory locust Locusta migratoria using egg pods that are artificially buried in the soil under outdoor conditions. Most eggs hatch in the mid‐morning, with a peak between 11.00 and 12.00 h, and none hatch before 09.00 or after 16.00 h. Furthermore, most egg pods complete hatching within a day, although some take 2 or 3 days, and egg hatching is interrupted by rain. There are no large differences in hatching time from May to September. Laboratory experiments in which the eggs are exposed to temperatures simulating outdoor conditions show that soil temperature is the main factor controlling hatching activity. The increase in temperature in the morning appears to trigger egg hatching, as confirmed by laboratory experiments, which may explain the similar hatching times between seasons. The seasonal patterns of temperature variation and hatching time suggest that the hatching time of L. migratoria eggs may be adjusted to allow the hatchlings to be exposed to high temperatures in the afternoon so that they can harden their bodies quickly.     

Combined effects of cadmium,copper and lead on developing herring eggs and larvae

Eggs of Baltic herring were incubated (10°C; 16S) in sea water containing mixtures of Cd, Cu and Pb at concentrations of 0.56–5.0, 0.0167–0.15, 0.56–5.0 mg metal/l; embryonic survival until hatching, viable hatch and uptake of metals by embryos and early larvae were measured. Negative effects of metals on embryonic survival and viable hatch were additive in the case of Cu and Cd. Pb did not exer detrimental effects. Uptake of metals with exposure time was non-linear in eggs and linear in larvae. Total uptake of Cu and Cd by eggs was subjected to antagonistic or synergistic action of the other two metals present. Accumulation of Pb by eggs was enhanced when Cu was also present.     

Effect of food concentration on the production and viability of resting eggs of the rotifer Brachionus: implications for the timing of sexual reproduction

1. Life‐table experiments with Brachionus calyciflorus test several hypotheses related to the idea that sexual reproduction in monogonont rotifers should occur when food resources are favourable. 2. The food concentration necessary for a fertilised mictic female to produce one phenotypically normal resting egg was higher than that for an amictic female to produce one daughter. At the lowest concentration of Cryptomonas erosa (1.25 × 10³ cells mL^?1), the lifetime fecundity of these two types of females was 0.9 and 1.4, respectively. 3. The lifetime fecundity of both fertilised mictic females and amictic females increased with food concentration to 3.4 resting eggs and 15.2 daughters female^?1, respectively. The approach to maximal fecundity with increasing food concentration was more rapid for fertilised mictic females, such that their lifetime fecundity relative to that of amictic females gradually decreased from 0.64 (at 1.25 × 10³ C. erosa mL^?1) to 0.22 (at 2.5 × 10⁴ C. erosa mL^?1). 4. The probability of a fertilised mictic female producing one or more abnormal resting eggs during her lifetime was high (approximately 75%). The mean proportion of abnormal eggs produced per female varied among the different food‐concentration treatments (26–38%) but was not higher at the low food concentrations. 5. The proportion of normal resting eggs that hatched was high (51–71%); those produced at low food concentrations were no less likely to hatch than those produced at high food concentrations. No abnormal resting eggs hatched. 6. The probability of a fertilised mictic female producing an abnormal resting egg increased rapidly with her age at all food concentrations. The probability of a normal resting egg hatching declined with maternal age at the low food concentration in one of two experiments. 7. The results support the idea that induction of mictic females should occur when food resources are good. Coincidence of sexual reproduction with low food availability risks low production of resting eggs for several reasons. Population size may be small, with a low probability of encounters between young mictic females and males, and fertilised mictic females may be unable to mature and produce resting eggs.     

The toxicity of mercuric chloride and methylmercuric chloride to Fundulus heteroclitus embryos in relation to exposure conditions

Synopsis Embryos in specific stage of the estuarine teleost, Fundulus heteroclitus, were exposed to mercuric chloride (MC) and methylmercuric chloride (MMC) under several distinct treatment conditions. Four-eight cell stage eggs (0-day old) were exposed for 4 days (continuous), 2 days and one day to each mercury compound. One-day old (mid-blastula), 2-day old (mid-neurula) and 5-day old (beating heart) embryos were exposed 4 days to MC and MMC. Mortality for the four days immediately following the initiation of exposure was the embryonic response measured. Under most exposure conditions to the 4–8 cell eggs, progressive and significant reductions in survival were observed at all concentrations above 40 and 30 gHg⁺⁺l^–1 as MC and MMC, respectively. Reducing the duration of exposure to 1 day most significantly increased the survival potential of the 4–8 cell eggs. For all exposure treatments to the 4–8 cell eggs, significant differences in survival, between eggs exposed to MC and MMC, were determined at 40, 60 and 80 gHg⁺⁺l^–1, indicating the presence of compound-dependent response differences. In all cases demonstrating response differences between MC and MMC exposed embryos, survival was significantly lower following exposure to MMC. Survival of embryos was progressively increased when the initiation of continuous exposure (4 days) was delayed 1, 2 and 5 days after fertilization. As a result, compound-dependent response differences were progressively shifted to higher He⁺⁺ concentrations. For both MC and MMC, survival of 1-day old embryos exposed for 4 days was greater than that of 0-day old eggs exposed for 1 day. Of the embryonic stages examined, it appears that the earlier cleavage stages are the most sensitive to mercury intoxication.